Google Voice is not a standalone telephone company. It is a call forwarding service. When someone calls your inbound Google Voice phone number, it will ring the linked, forwarding destinations you selected. Those destinations can be:

Up to six different 10-digit US telephone numbers.

OBi devices, which you authorized to access your Google account.

Any Hangouts client (on a laptop/desktop computer's web browser, or on an Android or iPhone), on which you have toggled-on the option to ring on inbound calls.

So: when someone calls number A, it is forwarding to your mobile phone number, C, over the regular mobile phone network. It is also forwarding to the Hangouts client you've signed into, with account A, on that phone.

You can decide: (1) ring that phone via Hangouts, or (2) ring it as a regular telephone call. If you enable both 1 + 2, it will be annoying and confusing, as both things will ring at once. If you want (1), then toggle the Hangouts setting to ring on inbound calls. If you want (2), then check-mark the box next to that mobile phone number in Google Voice settings. If you want the opposite, then remove the check mark or toggle off the Hangouts ringer.

If you ported number B into the same account as number A (per our other conversation here on this forum), then you can't treat those numbers separately. All inbound calls to either number A or number B will ring the phone or ring Hangouts in the same way.

That is how the service is designed to work. OBi devices are simply yet another call forwarding destination.

If you want to use numbers A and B separately, then you would need to transfer Google Voice number B to a second/different Google/Gmail account with Google Voice, and then sign onto account B on your smartphone to use it with Hangouts.